DEVONIAN SOURCE ROCKS IN THE MARTIMES BASIN, EASTERN CANADA

Widespread development of organic-rich deposits within the Maritimes Basin traditionally have been assumed to have commenced at the Devonian-Carboniferous boundary with the lacustrine to estuarine Horton Group (Tournaisian/early Mississippian). Two spatially disjunct formations, however, bear witness to earlier episodes of terrestrial organic-rich accumulation in the mid and late Devonian, representing some of the earliest fill of the Maritimes Basin: the McAdams Lake Formation (late Emsian-early Eifellian) in the south of the Sydney Basin and the Murphy Brook Formation (?Frasnian-Fammenian), which occurs as a fault-bound outlier within the Cobequid Highlands, south of the Cumberland Basin. The McAdams Lake strata lie within the oil window, whereas the Murphy Brook sediments are over mature, having been affected by tectonic and intrusive events along the Cobequid Fault zone. At Murphy Brook, this high maturation is problematic not only for hydrocarbon potential but also for palynology; past macrofloral studies however have identified an herbaceous Taeniocrada- Drepanophycus-Psilophyton-lycopsid macroflora.

The McAdams Lake Formation, site of oil shale exploration ca.1890, is estimated to be in the order of 1000 m thick. Palynology of the McAdams Lake Formation organics records a diverse terrestrial flora of vascular plants comparable with the type Eifelian strata of the Rhineland. The McAdams Lake source rocks sampled in this study reveal high TOC (4.28-17.33%), although a high percentage (37-62%) is inert organic matter (charcoal) which reduces their hydrocarbon generating capacity. Nonetheless, these gas-prone organic-rich shales, typified by Type III and rarely by Type II kerogen, are sufficiently organic-rich and of low maturity to warrant consideration in exploration and basin modelling. Organic petrography and Tmax of 435-451° suggests that vitrinite refectance (R0 0.57-0.70) of the organic-rich beds is suppressed by bitumen. Suppression notwithstanding, the curiously low maturity of the McAdams Lake shales in comparison to younger coals of the Sydney Basin suggests exhumation or only partial burial during the late Devonian or Carboniferous, which has implications for interpretations of the hydrocarbon potential of the Maritimes Basin and its evolution.